Attosecond Coherence Time Characterization in Hard X-Ray Free-Electron Laser

Area of Science:

• Physics
• Materials Science
• X-ray Science

Background:

• Characterizing the coherence time of ultra-fast hard x-ray pulses from free-electron lasers (FELs) is a significant challenge.
• This temporal coherence critically impacts x-ray-matter interaction processes.
• Conventional optical autocorrelation methods are impractical for hard x-rays due to the absence of suitable mirrors.

Purpose of the Study:

• To experimentally demonstrate a novel method for measuring the coherence time of ultra-fast hard x-ray pulses.
• To provide a universal diagnostic tool for temporal coherence applicable to various FEL parameters.

Main Methods:

• Utilizing a phase shifter to control the cross-correlation between the x-ray pulse and microbunched electrons.
• Experimental demonstration at the Linac Coherent Light Source (LCLS) for 6.92 keV FEL pulses.

Main Results:

• Successfully measured a coherence time of 174.7 attoseconds for the FEL pulses.
• The developed method is independent of general machine parameters.

Conclusions:

• The novel cross-correlation technique offers a viable solution for characterizing FEL pulse coherence time.
• This method is applicable across a wide range of photon energies, brightness, repetition rates, and pulse durations.
• The technique provides critical diagnostics for temporal coherence in x-ray FELs.